Xerographic plate of high quantum efficiency



Oct. 10, 1961 R. M. SCHAFFERT 3,

XEROGRAPHIC PLATE OF HIGH QUANTUM EFFICIENCY Filed Feb. 11, 1957' INVENTOR. ROLAND M. SCHAFFERT York Filed Feb. 11, 1957, Ser. N 0. 639,296 3 Claims. (Cl. 96-'1) My invention relates to xerog'raphy and more particularly to a sensitive plate therefor.

In xerography it is usual to form an electrostatic latent image on a surface. One method of doing this is to charge a photoconductive insulating surface and then dissipate the charge selectively by exposure to a pattern of activating radiation. Other means of forming electrostatic latent images are set forth in US. 2,647,464 to James P. Ebert. Whether formed by these means or any other, the resultant electrostatic charge pattern is conventionally utilized by the deposition of an electroscopic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopic particles corresponding to the electrostatic latent image. Alternatively, the electrostatic charge pattern may be transferred to an insulating film and the electroscopic particles deposited thereon to form the visible image. In any case, this visible image in turn may be transferred to a second surface to form a xerographic print or may be fixed directly to the photoconductive surface.

Most present commercial applications of xerography are directed to the reproduction of printed and pictorial information under controlled conditions wherein the amount of light available may be closely regulated. For direct outdoor photography and other uses where the useful light may not be conveniently controlled, one requires a xerographic plate whose sensitivity or' speed compares favorably with that of the high speed negative films of silver halide photography. The speed of present commercial xerographic selenium plates is roughly equivalent to an ASA rating of 4. In contrast to this, black-and-white photographic film has a speed of at least about 50 and even color film has a speed of the order of at least about ASA. There thus exists a strong demand to increase the speed of xerographic plates to at least equal the ASA rating of present color films and, if possible, to materially exceed even this speed. It is an object of this invention to present means and apparatus to accomplish this objective.

In accordance with my invention, theefifective speed of Xerographic plates is increased through the intermediary effect of a layer of a suitable electroluminescent material between the photoconductive insulating material and the conductive backing of a xerographic plate.

In the accompanying drawings FIGURE 1 shows a cross section of a xerographic plate according to one embodiment of the invention.

FIGURE 2 is a cross section of a xerographic plate according to another embodiment of the invention.

The xerographic plate ordinarily used in xerography comprises a conductive backing having coated thereon a photoconductive insulating material. The conductive backing may comprise any conductive material as aluminum, brass, zinc, chromium, paper, glass having a cone ductive coating as of tin oxide,- etc. Suitable photoconductive insulating materials include continuous layers of materials such as an'thracene, selenium, sulfur, mixtures of selenium with tellurium, arsenic, sulfur, and so on. Alternatively, the photoconductive insulating material may comprise a suitable inorganic pigment phosphor material such as an oxide, sulfide or selenide of zinc or cadmium finely dispersed in an insulating resin binder such as a vinyl resin, a cellulose ether or ester, a silicone resin, etc. Whatever the nature of the photoconductive insulating layer, this layer in turn may, if desired, be covered by a transparent insulating coating as of vinyl resin, silicone resin, chlorinated rubber, cellulose ether or ester, etc., to protect the surface thereof from abra= sion and mechanical damage.

Referring to FIGURE 1, a xerog'raphic plate 10 according to the instant invention may comprise a base material 11 transparent to the radiation used (as, for instance, glass in the case of visible light, quartz in the case of ultraviolet, and metal in the case of X-ray radiation), is provided with a conductive coating 12 ca at least one surface-thereof, which coating 12 is also transparent to the radiation used. In the case of a glass or quartz base 11, a thin layer of tin oxide, indium oxide, aluminum, silver, etc., may be readily utilized for forming such transparent electrically conductive coatings. On this conducting film 12 is deposited a thin layer 13 of electroluminescent phosphor. The phosphor is deposited as a continuous layer as by vacuum evaporation, chemical deposition techniques, etc.

The particular type of electroluminescent phosphor useful herein is that which achieves appreciable light amplification when subjected to the combined stimulus of imposed radiation and a suificiently large D.C. field and hence are termed DC-enhanced electroluminescent phosphors. Such a phosphor may be made by effecting areaction between zinc vapor and hydrogen sulfide in the presence of manganese and chlorine to form a zinc sulfide: manganese chloride film on the surface to be coated. Such a film fluoresces weakly when subjected to ultraviolet radiation. However, when a sufficiently large D.C. field is impressed across such a film in the presence of the radiation, about ten photons of visible light are produced for each incident ultraviolet photon.

On top of the stratum of electroluminescent material 13 is placed a layer of photoconductive insulating material 14. -If selenium, anthracene or other material is used as the photoconductive insulator, the photoconduc= tor may be sprayed on the desired surface in molten form or evaporated onto the plate under high vacuum. Likewise, the selenium, anthracene and inorganic pigment phosphor materials may all be applied in the form of a'lacquer by forming a dispersion of the finely-ground photoconductor in a suitable film-forming resin binder.

In operation electrical charges suflicient to raise the electrical field through layer 13 to near the breakdown point are applied to the surface of layer 14 as by corona discharge as set forth in copending application Serial No. 154,295, filed April 6, 1950 by L. E. Wal-kup or other means. The image which it is desired to reproduce is now projected (using a conventional projection optical system) onto layer 13 through layers 11 and 12 using ultraviolet light. The ultraviolet rays will be absorbedin layer 13, releasing electrons to the conduction band. Due to the high electrical field these electrons will be accelerated and as they move through the layer 13 will excite other electrons by collision or excition transfer, giving up sufficient energy to raise these electrons to the conduction band. The new electrons in turn will be accelerated, releasing more electrons. Thus, an ava- Ianche orquantum multiplication takes place. Many of these excited electrons will fall into illuminescent or inn purity centers, causing light to be emitted. Some electrons may pass through layer 13 and be injected into layer 14.

Two phenomena will then contribute to increase the rate of decay of potential across layer 14. First, the light image itself will be amplified in layer 13 so that the light entering layer 14 will be many times more intense that that of the original image. Second, current carriers excited in layer 13 and passing through layer 13 will be injected into layer 14 amplifying the photocurrent through layer 14.

The light emitted within layer 13 will not necessarily be of the same wave length as that of the incident light image, but will be characteristic of the activator centers in layer 13.

In the areas where the light excites layer 13 significant conductivity is imparted to the photoconductor 14 in the manner described. As a result, the charges on the surface of the photoconductor '14 immediately opposite the point of incidence of the light are selectively dissipated in accordance with the intensity of the incident light. There will then result on the surface of the photoconductor an electrostatic charge pattern corresponding to the original to be reproduced. This image may then be made visible using any of the methods known to those skilled in the art, as, for example, cascade development as shown in U.S. 2,018,552, to E. N. Wise, powder cloud development as shown in U.S. 2,725,304 to Landrigan et al., etc. If desired this powder image may be transferred to paper, plastic, or other recording means as shown for example in U.S. 2,576,047 to R. M. Schaffert or in U.S. 2,661,289 to C. R. Mayo et a1. and fixed thereto as shown in U.S. 2,701,765 to J. Codichini et al. After cleaning any residual powder from surface 14, as by gentle swabbing with cotton, the plate may be used to repeat the cycle.

Another embodiment of the invention is shown in FIGURE 2. This differs from the embodiment of FIG- URE 1 in that an extra conductive transparent layer 15 is interposed between the D.C.-enhanced electroluminescent phosphor layer 13 and the photoconductive layer 14. In this case the D.C. field is maintained across the electroluminescent layer 13 by suitable D.C. voltage source 16 applied between conductive transparent layers 12 and 15 when switch 17 is closed. Hence, the application and magnitude of the applied D.C. field impressed across the electroluminescent layer 13 is largely independent of the charge applied to the surface of the photoconductor 14. In this case the dissipation of an uniform layer of charge placed on the surface of the photoconductor 13 will be dependent not only on the length of exposure to the activating radiation, but also to the duration of the impressed D.C. potential. The operation of the plate is the same as that of the plate shown in FIGURE 1 except that it is desirable that the activation and deactivation of the D.C. potential as by closing and opening switch 17 be so ordered as to coincide with the exposure to the light image.

Plates according to the instant invention may be in flat, spherical, cylindrical or other conformations as desired.

The novel plates according to the instant invention are useful in all of the processes, methods and apparatuses wherein xerographic plates are normally usedthe instant plates differing only in having a substantially greater response to activating radiation than plates prepared according to the prior art. If desired, the photoconductive insulating layer 14 may be protected by a thin insulating coating as described above.

If desired, the D.C.-enhanced electroluminescent mate rial 13 may be tailored as by proper selection of the material providing the activator centers to emit light of a relatively narrow band of wavelengths while at the same time absorbing light throughout the visible range of the spectrum. Thus, absorbed light in layer 13 will trigger the avalanche described above with light ranging from red through blue or ultraviolet whereas the emitted light would be essentially in one region, such as blue. Using a D.C.-enhanced electroluminescent layer such as this would permit the use of photoconductive insulating layers 14 whose photoresponse is confined to the region of emission of layer 13. This would provide a panchromatic xerographic plate even when layer 14' has essentially monochromatic response. Thus, the photoconductive insulating material 14 would be selected primarily in terms of its photosensitivity rather than in terms of photoresponse, relying on layer 13 to provide the desired photoresponse for the over-all plate. Other variations or modifications of the instant invention will be apparent to those skilled in the art.

The examples given above are in illustration only and not in limitation of the invention, references being bad to the appended claims for this purpose.

I claim:

1. The method of producing an electrostatic latent image corresponding to an optical image comprising the following steps:

A. Applying a D.C. electric potential between two transparent conductive layers which have a layer of electroluminescent phosphor material therebetween which luminesces only under the combined stimulus of a D.C. electric field and activating radiation;

B. Placing an uniform electrostatic charge on the outer surface of a layer of photoconductive insulating material which is supported by on of said transparent conductive layers;

C. Subjecting said phosphor layer to an optical image by projecting said optical image onto said phosphor layer through the said transparent conductive layer which is free from said photoconductive insulating material whereby the resulting changes in electric field distribution produce an electrostatic latent image on the outer surface of said photoconductive insulating material; and

D. Removing said electric potential when said electrostatic latent image is formed.

2. The method according to claim 1 wherein the phosphor is a manganese chloride activated zinc sulfide.

3. The method according to claim wherein l the phosphor is a manganese chloride activated Zinc sulfide and the photoconductive insulating material is vitreous selenium.

References Cited in the file of this patent UNITED STATES PATENTS 2,354,109 Flood July 18, 1944 2,650,310 White Aug. 25, 1953 2,654,853 Weimer Oct. 6, 1953 2,687,484 Weimer Aug. 24, 1954 2,698,915 Piper Jan. 4, 1955 2,764,693 Jacobs et a1. Sept. 25, 1956 2,776,367 Lehovec Jan. 1, 1957 2,798,959 Moncrief-Yeates July 9, 1957 2,798,960 Moncrief-Yeates July 9, 1957 2,833,648 Walkup May 6, 1958 2,844,493 Schlosser July 22, 1958 2,844,734 Hartmann July 22, 1958 2,886,434 Owens May 12, 1959 FOREIGN PATENTS 157,101 Australia June 16, 1954 OTHER REFERENCES Williams: Amer. Jour. Roentgenology, vol. 75, No. 1, pp. 77-82 (1956).

Wainer: Photo. Eng, vol. 3, No. 1952, pp. 1 to 20.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No; 3,003,869 I I October 10,

Roland M. Schaffert I It is hereby certified that error appears in the above numbered pat entrequiring correction and that the said Letters Patent should read as corrected below.

Column 4 line 28,- for "on" read one Signed and sealed this 10th day of April 1962.

(SEAL) Attest:

ERNEST W. ASWIDER Attesting Officer DAVID L. LADD Commissioner of Pat 

1. THE METHOD OF PRODUCING AN ELECTROSTATIC LATENT IMAGE CORRESPONDING TO AN OPTICAL IMAGE COMPRISING THE FOLLOWING STEPS:
 4. APPLYING A D.C. ELECTRIC POTENTIAL BETWEEN TWO TRANSPARENT CONDUCTIVE LAYERS WHICH HAVE A LAYER OF ELECTROLUMINESCENT PHOSPHOR MATERIAL THEREBETWEEN WHICH LUMINESCES ONLY UNDER THE COMBINED STIMULUS OF A D.C. ELECTRIC FIELD AND ACTIVATING RADIATION,
 8. PLACING AN UNIFORM ELECTROSTATIC CHARGE ON THE OUTER SURFACE OF A LAYER OF PHOTOCONDUCTIVE INSULATING MATERIAL WHICH IS SUPPORTED BY ON OF SAID TRANSPARENT CONDUCTIVE LAYERS, C. SUBJECTING SAID PHOSPHOR LAYER TO AN OPTICAL IMAGE BY PROJECTING SAID OPTICAL IMAGE ONTO SAID PHOSPHOR LAYER THROUGH THE SAID TRANSPARENT CONDUCTIVE LAYER WHICH IS FREE FROM SAID PHOTOCONDUCTIVE INSULATING MATERIAL WHEREBY THE RESULTING CHANGES IN ELECTRIC FIELD DISTRIBUTION PRODUCE ON ELECTROSTATIC LATENT IMAGE ON THE OUTER SURFACE OF SAID PHOTOCONDUCTIVE INSULATING MATERIAL, AND D. REMOVING SAID ELECTRIC POTENTIAL WHEN SAID ELECTROSTATIC LATENT IMAGE IS FORMED. 